"Fred McKenzie" wrote in message
...
In article ,
Jeff Liebermann wrote:


I hate to admit it, but I think I've built them backwards over the
years. Maybe that's why J-poles have such a lousy reputation and why
I think they suck?

Jeff-

Please pardon my ignorance. I understood that a J-Pole is a half wave
antenna connected to a quarter wave stub. The transmission line is
connected to a low impedance point (50 Ohms?) on the stub.

In the literature, it is usually fed as an unbalanced antenna, but it is
not. Therefore, it should not matter which side has the center
conductor or shield - they are both wrong!

Fred
K4DII

I acknowledge the split opinion on the feed. I've often thought it
shouldn't matter, since the idea is to excite the stub (which is, itself, a
half-wave if you count both sides). I follow the crowd, frankly.

How would you change the feed method? I've had generally good performance
from my J-poles but I'll gladly improve what I do, if you have some ideas.

73,
"Sal"
(KD6VKW)

In article ,
Sal salmonella@food poisoning.org wrote:

How would you change the feed method? I've had generally good performance
from my J-poles but I'll gladly improve what I do, if you have some ideas.

One of the older ARRL guides or antenna books shows a balanced method
of feeding a J-pole. A standard half-wave coaxial balun is used. The
two balanced outputs of the balun are tapped onto the two sides of the
J-pole matching section, some distance above the usual "50-ohms-or-
thereabouts" attachment point.

My understanding is that the impedances "seen" on the two sides of the
matching section won't be identical; the short side ends at an
open-circuit point and the other side "ends" at the beginning of the
half-wave section, where the impedance is high but not quite an open
circuit. Hence, you won't achieve complete balance this way - there
will probably be some current flow on the outside of the halfwave coax
balun section.

"David Platt" wrote in message
...
In article ,
Sal salmonella@food poisoning.org wrote:

How would you change the feed method? I've had generally good performance
from my J-poles but I'll gladly improve what I do, if you have some ideas.

One of the older ARRL guides or antenna books shows a balanced method
of feeding a J-pole. A standard half-wave coaxial balun is used. The
two balanced outputs of the balun are tapped onto the two sides of the
J-pole matching section, some distance above the usual "50-ohms-or-
thereabouts" attachment point.

My understanding is that the impedances "seen" on the two sides of the
matching section won't be identical; the short side ends at an
open-circuit point and the other side "ends" at the beginning of the
half-wave section, where the impedance is high but not quite an open
circuit. Hence, you won't achieve complete balance this way - there
will probably be some current flow on the outside of the halfwave coax
balun section.

I don't know which one would be the best but I have seen 3 methods of
feeding the J-pole. If you insulate the bottom then you hook the feedline
to the bottom with the center of the coax to the long side. If you do not
insulate the bottom you tap up the matching segment so that you get a 50 ohm
(if that is the coax used) match with the center of the coax connected to
the long leg. Then there is the balun made out of coax that is hooked up to
the matching segment so that a low swr is obtained.



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Don't forget the Arrow Open Stub J-pole.

http://www.arrowantennas.com/osj/j-pole.html

In article ,
W5DXP wrote:

Don't forget the Arrow Open Stub J-pole.

http://www.arrowantennas.com/osj/j-pole.html

There is (or was) a nice writeup of this variety on Cebik's web
site. He refers to it as a "variant J-pole". The feed arrangement is
different (it's open at the feedpoint, rather than being fed a few
inches above a short), and the arm lengths are different than with a
"classic" J-pole. The current distributions are different, of course,
but the radiation pattern is only very slightly different than the
"classic" variety.

I have one of these dual-band types in my "go-kit" (it's actually a
knock-off, built locally based on the plans that Arrow used to have on
their web site) and it's served me well. I usually mount it at the
top of a two-or-three-segment aluminum mast, sitting in an old hefty
movie-camera tripod base.

Much too large and heavy for a vehicle, of course, but I imagine you
could build a somewhat-similar open-stub J-pole using much lighter
materials (e.g. fiberglass whips with wire fastened inside or
outside).

On Tuesday, February 18, 2014 6:26:40 PM UTC-6, David Platt wrote:
The feed arrangement is
different (it's open at the feedpoint, rather than being fed a few
inches above a short), and the arm lengths are different than with a
"classic" J-pole.

If one will ignore the long 2m element and draw a schematic of it just for the 70cm band, one will realize that it is actually just a standard Zepp antenna.

In article ,
(David Platt) wrote:

In article ,
W5DXP wrote:

Don't forget the Arrow Open Stub J-pole.

http://www.arrowantennas.com/osj/j-pole.html

I have one of these dual-band types in my "go-kit" (it's actually a
knock-off, built locally based on the plans that Arrow used to have on
their web site) and it's served me well.

David-

Using the Wayback Machine https://archive.org/index.php, I was able to
find the old Arrow plans listed under instruction sheets.

Then I found that the latest version was a single PDF file that has the
same information!

At the recent Orlando Hamfest, someone was selling dual band J-Pole
copies of the Arrow. Now I wish I had bought one.

Fred

On Tue, 18 Feb 2014 18:10:39 -0500, "Ralph Mowery"
wrote:

I don't know which one would be the best but I have seen 3 methods of
feeding the J-pole. If you insulate the bottom then you hook the feedline
to the bottom with the center of the coax to the long side. If you do not
insulate the bottom you tap up the matching segment so that you get a 50 ohm
(if that is the coax used) match with the center of the coax connected to
the long leg. Then there is the balun made out of coax that is hooked up to
the matching segment so that a low swr is obtained.

There's also the American Legion J-Pole or the Silicon Valley
Emergency Communications Systems J-Pole:
https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396072666399154
https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396111364421106
This design does one thing right that none of the other J-Pole
mutations seem to consider. The length of the wire between the coax
connector center conductor and the driven element is an inductor. In
order to tune out this inductance, one needs a series capacitor, with
the inductor and capacitor tuned to the operating frequency. In other
words, a gamma match. The series capacitor is formed by the insulated
turns of electrical wire wrapped around the driven element.

Another thing this design does right is use the zero current point at
the bottom of the antenna as a ground. The problem is that it also
extends the length of the center wire, which makes using a gamma match
all the more important. I think putting the 50 ohm feed point and the
corresponding ground close to each other were either to reduce the
inductance of the connecting wire, or some manner of mutation from
when it was fed by a balance line.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

"Jeff Liebermann" wrote in message
...


... The length of the wire between the coax
connector center conductor and the driven element is an inductor. In
order to tune out this inductance, one needs a series capacitor, with
the inductor and capacitor tuned to the operating frequency.

Yes.

I've been making copper pipe and 2-wire transmission line J-poles for almost
20 years. With pipe, I usually fasten the two feed points with clamps and
slide the connections up and down. I'll get a VSWR low-point in-band but
early-on, I discovered that the best VSWR was often about 1.7:1. I had read
about (but never built) a gamma match, so I'd heard about the series cap to
tune out the inductance. I tried a series cap at the feed and it helped.

70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading
somewhere in the band. Does such a 0.3 dB improvement matter? That's not
my call. When I'm essentially playing with the technology, I can take more
time than if I'm working, like to a deadline or a budget.

"Sal"

On Wed, 19 Feb 2014 21:16:19 -0800, "Sal" salmonella@food
poisoning.org wrote:

"Jeff Liebermann" wrote in message
.. .
... The length of the wire between the coax
connector center conductor and the driven element is an inductor. In
order to tune out this inductance, one needs a series capacitor, with
the inductor and capacitor tuned to the operating frequency.

Yes.
I've been making copper pipe and 2-wire transmission line J-poles for almost
20 years. With pipe, I usually fasten the two feed points with clamps and
slide the connections up and down. I'll get a VSWR low-point in-band but
early-on, I discovered that the best VSWR was often about 1.7:1. I had read
about (but never built) a gamma match, so I'd heard about the series cap to
tune out the inductance. I tried a series cap at the feed and it helped.

70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading
somewhere in the band.

I'm not sure of the frequency of your J-pole. The inductance of about
6 cm of #12AWG solid wire is about 0.05 uH.
http://www.consultrsr.com/resources/eis/induct5.htm
To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be
about 200 pf.

Does such a 0.3 dB improvement matter? That's not
my call. When I'm essentially playing with the technology, I can take more
time than if I'm working, like to a deadline or a budget.

0.3dB is about 6.7% loss. Probably not important or toss a coin?
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558